During recent years, microsurgical techniques have become increasingly common in intracranial operations, such as those involving vascular malformations, aneurysms and certain tumors, e.g., acoustic neuromas or hypophyseal tumors. In neurosurgery, during most intracranial procedures, a microscope is used to allow the surgeon to work on structures that are located deep within the brain. Under these circumstances, retraction of intervening brain tissue is generally required for adequate exposure of the target brain tissue. Numerous brain retractors of various sizes and shapes have been developed for this purpose. See FIGS. 1 and 2.
As a consequence of their function, all brain retractors must deliver some degree of pressure to the engaged brain tissue. If this pressure exceeds some level, tissue damage can occur. Depending on the degree of the tissue damage, this can result in a prolonged hospital stay, an increase in medical costs and, in many cases, impairment of neurological functions, psychiatric changes, and loss of quality of life.
More particularly, when the retractor engages the brain tissue, pressure is imposed on the engaged brain tissue, thereby causing some degree of tissue deformation and partial or total closure of blood vessels, which in turn impairs oxygen delivery to brain cells. The severity of the damage to the brain depends on various factors such as the Brain Retraction Pressure (BRP) distribution, the geometry of the brain retractor, the geometry and physical properties and type of brain tissue, the vascular pressure, the duration of the retraction, etc. In this respect it is noted that approximately 50,000 brain surgeries are performed in the United States each year which last between 30 and 45 minutes. Various studies indicate that brain tissue is likely to be negatively affected after 15 minutes of excessive retraction pressure.
Among other things, it has been found that if the Mean Arterial Pressure (MAP) exceeds the Brain Retraction Pressure (BRP) by less than 70 mm Hg, the brain will be damaged (i.e., brain damage will occur if MAP-BRP<70 mm Hg). However, it has also been found that when the difference between BRP and MAP is greater than 100 mm Hg, the brain will typically recover completely (i.e., no lasting brain damage will occur when MAP-BRP>100 mm Hg).
Furthermore, it has been found that the Regional Cerebral Blood Flow (RCBF) should be greater than 10-13 ml/100 gm/minute, otherwise there is an imminent danger of focal ischemic brain damage.
Thus, to safeguard the brain during a lengthy surgery, it is desirable that the brain retractor be provided with a means for monitoring the pressure applied to the brain tissue and for warning the surgeon if and when that pressure exceeds a maximum level.
Tissue retractors are also used in medical fields other than neurosurgery. By way of example, tissue retractors are commonly used in bladder procedures, liver vascular surgery, gynecology, organ transplantation, etc. To safeguard these other tissues during such procedures, it is also desirable that such other tissue retractors be provided with a means for monitoring the pressure applied to the tissue and for warning the surgeon if and when that pressure exceeds a maximum level.